1. Field of the Invention
The invention relates to an image processing apparatus and method for executing a color space compressing process to image data.
2. Related Background Art
A conventional color space compressing method is mainly classified into three methods as shown in conceptual diagrams and characteristics diagrams of FIGS. 13A to 13C.
Method of FIG. 13A
A method whereby the whole color is changed so that an input color signal out of a color reproduction range enters the range, thereby mapping into the color reproduction range.
Method of FIG. 13B
A method whereby the colors within the color reproduction range are not changed but the colors out of the range are mapped to a surface of the color reproduction range.
Method of FIG. 13C
A method whereby a faithful color reproduction range for executing a faithful color reproduction and a mapping color reproduction range for mapping the colors out of the faithful color reproduction range are provided within the color reproduction range and a mapping boundary serving as a boundary of the two ranges is changed in accordance with a ratio of the total number of pixels and the number of pixels out of the color reproduction range.
According to the method of FIG. 13A, since the relative relation among the colors is maintained, there is an advantage such that a color balance is not broken. On the other hand, however, there is a drawback such that the total color changes.
According to the method of FIG. 13B, although there is an advantage such that the colors within the color reproduction range are faithfully reproduced, there is a drawback such that the reproduction of the colors out of the range is limited to a size of the surface area of the color reproduction range, so that a gradation is lost.
The method of FIG. 13C keeps the advantages of the methods of FIGS. 13A and 13B and compensates for their drawbacks. However, in a case of a linear mapping, there is a drawback such that a continuity on a mapping boundary is not maintained.
The conventional color space compressing method has the following drawbacks.
(1) When the number of colors out of the color reproduction range is small, if the color space compressing method of FIG. 13A is used, the total color changes.
(2) When the number of colors out of the color reproduction range is large, if the color space compressing method of FIG. 13B is used, the color reproduction out of the range is limited to the size of surface area of the color reproduction range, so that a gradation is lost.
(3) In case of a gradation including the colors out of the color reproduction range from the inside of the color reproduction range, if the color space compressing method of FIG. 13C using the linear mapping is used, the color becomes discontinuous at a mapping boundary.